Microwave-attenuating composite materials, methods for preparing the same, intermediates for preparing the same, devices containing the same, methods of preparing such a device, and methods of attentuating microwaves

ABSTRACT

The present invention provides microwave attenuating, filled composite materials which contain a polymer or ceramic matrix and metallic tubules and processes for making the same and devices which contain such materials.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to microwave-attenuating compositematerials. The present invention also relates to methods for preparingsuch microwave-attenuating composite materials. The present inventionalso relates to intermediates useful for preparing suchmicrowave-attenuating composite materials. The present invention furtherrelates to devices containing such microwave-attenuating compositematerials and methods for preparing such devices. The present inventionadditionally relates to methods for attenuating microwaves. Morespecifically, the present invention relates to methods for absorbingmicrowave radiation, for signal attenuation at appropriate wavelengths,and for isolating transmitting antennas and receiver components in radarsets.

2. Discussion of the Background

Electronic devices are becoming increasingly compact and complex. Atmicrowave frequencies, cross-talk and ringing are becoming seriousproblems that need to be addressed. In particular, devices such ascellular telephones, pagers, and palm- or lap-size computers containmany circuits that require isolation from each other for maximumperformance. In addition, military platforms such as ships and aircraftoperate a variety of radio and radar systems that may interfere witheach other, thus requiring isolation measures. These systems may consistof passive devices, active devices, or mixtures thereof. The number,sophistication, and cost of these systems continue to rise, while thedemand for improved performance moves forward at an even a faster pace.

In order to prevent cross-talk, it is necessary to isolate thecomponents of electronic devices. This may be accomplished by placing amaterial of defined and controllable shape, which absorbs interferingfrequencies of radiation, between the components. However, the gap wherethe isolating material is placed may be small, complex in shape, andhard to reach. In addition, the isolating material is preferablydimensionally stable, compact and lightweight, chemically inert andmechanically robust, and unaffected by its operating environment.

At present, numerous microwave-absorbing composite materials are madefrom organic matrix resins and carbonyl iron powder. Typically, thecarbonyl iron powder has a density near 7.9 g/cm³, and the composite hasa density near 5 g/cm³. The high density of the conventionalmicrowave-absorbing materials presents the serious drawback of impartinga dramatic increase in the weight of any device incorporating such amaterial, which is especially undesirable in portable, or hand-helddevices, such as mobile telephones, lap-top computers, or aircraft.

Moreover, conventional microwave-attenuating materials are not aseffective as desired. In this regard, it is noted that on a commercialor industrial scale even small improvements in absorbing microwaveradiation, signal attenuation, and isolation of signal devices areeconomically significant.

Therefore, there remains a critical need for microwave-attenuatingcomposite materials, which do not suffer from this drawback. There alsoremains a need for methods for preparing such microwave-attenuatingcomposite materials and intermediates useful for preparing suchmicrowave-attenuating composite materials. In addition, there remains aneed for devices, which contain such a microwave-attenuating compositematerial, and methods for preparing such devices. There also remains aneed for improved methods of attenuating microwaves.

SUMMARY OF THE INVENTION

Accordingly, it is one object of the present invention to provide novelcomposite materials, which are useful for attenuating microwaves.

It is another object of the present invention to provide novel methodsfor preparing such composite materials.

It is another object of the present invention to provide novelintermediates which are useful for preparing such composite materials.

It is another object of the present invention to provide novel devices,which contain such a composite material.

It is another object of the present invention to provide novel methodsfor preparing such devices.

It is another object of the present invention to provide novel methodsfor attenuating microwaves.

These and other objects, which will become apparent during the followingdetailed description, have been achieved by the inventors' discoverythat composite materials, which comprise:

(a) a polymer or ceramic matrix; and

(b) a plurality of metal microtubules dispersed within said matrix, areeffective for the attenuation of microwaves.

The inventors have also discovered that such composite materials may beprepared by a method comprising:

(1) incorporating a plurality of metal microtubules in a polymer orceramic matrix.

The inventors have also found that such composite materials may beprepared from an intermediate, which comprises:

(a) a polymer matrix precursor or a ceramic matrix precursor; and

(b) a plurality of metal microtubules dispersed within said polymermatrix precursor or said ceramic matrix precursor.

The inventors have also discovered that electronic devices, whichcomprise a microwave-attenuating composite, said composite comprising

(a) a polymer or ceramic matrix; and

(b) a plurality of metal microtubules dispersed within said matrix,exhibit a reduction in the problems associated with or caused byineffective attenuation of microwaves.

The inventors have additionally found that such devices may be preparedby a process, which comprises:

(1) incorporating a microwave-attenuating composite material in anelectronic device,

wherein said microwave-attenuating composite material comprises:

(a) a polymer or ceramic matrix; and

(b) a plurality of metal microtubules dispersed within said matrix.

The inventors have also found that microwaves may be effectivelyattenuated between a source point and a detection point by a method,which comprises:

(1) placing a microwave-attenuating composite material between saidsource point and said detection point, wherein saidmicrowave-attenuating composite material comprises:

(a) a polymer or ceramic matrix; and

(b) a plurality of metal microtubules dispersed within said matrix.

As will be recognized from the following detailed description, theflexibility, density, frequency response, and environmental stability ofthe present microwave-attenuating composite materials may be varied asdesired over wide ranges.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Thus, in the first embodiment, the present invention provides compositematerials, which comprise:

(a) a polymer or ceramic matrix; and

(b) a plurality of metal microtubules dispersed within said matrix,which are effective for the attenuation of microwaves.

In the context of the present invention, the term “microwaves” refers toelectromagnetic radiation having a frequency of 100 MHz to 200 GHz.

In the case of a polymer matrix, the matrix may be formed of anysuitable polymer material. A wide variety of polymers may be used as thematrix resin in this invention. These include but are not limited tovinyl, styrene-butadiene, natural rubber, nitrile rubber, and acrylicresins, in addition to a polyurethane, an epoxy, or a siloxane. Thesepolymers may be used singly or in combination; when used in combinationthe amount of each may be adjusted to optimize the performance of thecomposite. In general, the polymer matrix resin may be obtained bypremixing the base component, followed by the addition of a curingagent. Preferred curing agents include diisocyanate prepolymerscontaining a plasticizer, polyoxypropylenediamine, and copolymers ofmethylhydrosilane and dimethylsiloxane. In this case, none of theingredients of the resin are volatile, and shrinkage during curing isless than 0.1%. Although, each of the resins cures at room temperature,heating the liquid mixture to 60° C. may accelerate curing.

Especially preferred elastomeric polymers include epoxy resins andpolyurethanes.

In the case of a ceramic mixture, the matrix may be likewise anyconventional ceramic, provided that the ceramic matrix may be formedfrom a mixture of ceramic matrix precursor(s) and metal microtubulesunder conditions which do not destroy the desired attenuation. Suitableceramics are described in Kirk-Othmer, Encyclopedia of ChemicalTechnology, 4^(th) Ed., Wiley Interscience, NY, vol. 5, pp. 599-728(1993), which is incorporated herein by reference. Ceramics which areprepared from a sol/gel precursor(s) are particularly preferred.

The term metal microtubule refers to any metal or metallized cylinderhaving a length of 2 to 200 micrometers (μm), preferably 5 to 100 μm,more preferably 10 to 30 μm; an outside diameter of 0.5 to 3.0 μm,preferably 0.5 to 2.0 μm, more preferably 1.0 to 2.0 μm; an insidediameter of 0 to 0.5 μm, preferably 0.01 to 0.5 μm, more preferably 0 to0.4 μm, even more preferably 0.01 to 0.4 μm, still more preferably 0 to0.3 μm, even more preferred 0.01 to 0.3 μm. Preferably, the metalmicrotubule is electrically conductive and resistant to oxidation. Inthis case, the use of microtubules with a Ni overcoat to protect againstoxidation is preferred.

The preparation of such metal or metallized microtubules is described indetail in U.S. Pat. Nos. 4,911,981; 5,049,382; 5,342,737; 5,814,414; and6,280,759, all of which are incorporated herein in their entirety byreference.

Two types of microtubules, lipid and halloysite, are especiallypreferred for use in the present invention. Lipid microtubulesself-assemble from water-alcohol solutions of biologically-deriveddiacetylenic lipids and precipitate at the appropriate temperature andconcentration. Electroless metallization processes deposit anelectrically conductive coating of copper, nickel, iron, otherelectroless metals, or combinations thereof upon the lipid microtubules.The lipid may then be withdrawn from the center of the microtubule andreused. The resulting metallized microtubules have high aspect ratios(20-200), low density, and impart appreciable permittivity when used inorganic matrix resins. These materials are described more fully in U.S.Pat. No. 6,013,206, which is incorporated herein by reference in itsentirety.

A further method to form the electroless-plated metallic microtubules isto process the lipid microtubules in such a manner to yield a suspensionof the lipid at a concentration of about 2.5 mg/ml in a mixed solventconsisting of about 70% of an alcoholic mixture and about 30% distilledwater, wherein the alcoholic mixture consists of about 20 to 80%methanol and about 20 to 80% ethanol. Once formed, the microtubules aresubjected to a polyanion mixture, such as polystyrenesulfonate, at adilution of 0.1 to 1.0 mg/ml in distilled water.

The polyanion is allowed to bind to the lipid microtubules for 15 to 45minutes. After the incubation period, the mixture is diluted with anequivalent volume of a polycation mixture, such as polyethyleimine or achitosan, at a dilution of 0.5 to 1.5 mg/ml. It maybe advantageous tocross-link the polycationic layer to promote stability by the use ofgluteraldehyde, or another suitable crosslinking agent, atconcentrations sufficient to cross-link the polyion layers.

In order to prevent the accumulation of an excess amount ofpolyelectrolyte and therefore excess bundling and bridging of themicrotubules it is necessary to use a slightly deficit amount of thepolyion so that the polyion in solution will be 100% bound to the tubulesurface and not found in excess in the supernatant solution. Oneapproach to calculate the amount of polyion to be added is to determinethe total surface charge available for binding by titrating the tubulesurface with a charged dye complex such as Toluidine Blue O for thedetermination of the net negative charge available or the use of a dyesuch as FastUSOL Red 50L for polyanions. In order to facilitate thetitration, a known quantity of the tubule suspension or dispersion istitrated with a known dilution of the dye stuffs such that the dye isadded to the tubule/polyion suspension and allowed to bind for 15minutes. Then the tubules may be spun out of solution at 2000 g and thesupernatant is analyzed by visible spectrophotometry for the presence ofunbound dye in solution. The titration is complete when the dye is foundto remain in solution following the binding period. The amount of dyeadded is calculated and the amount of charge determined. Then theappropriate amount of the oppositely charged polyion is added to thesolution and allowed to bind, and the process is repeated until therequired number of layers is obtained.

The microtubule suspension is then allowed to settle by gravity or maybe filtered to remove the excess polyion solution and the excessalcohol. However, this step is not necessary, if the concentration ofalcohol at this point is insufficient to precipitate the metal salts.The microtubules are then resuspended in distilled water to the originalvolume used for microtubule production. The final concentration of lipidshould be 2.0 to 2.5 mg/ml. A catalytic mixture of palladium salts isadded at a volume ratio of 1:500 of catalyst solution or dispersion tomicrotubule suspension. The catalyst is allowed to bind to the polyioncomplexes for a minimum of 30 minutes followed by immediate plating.

The resultant catalyzed microtubules may be electroless coated withsimple formulations of copper, nickel, cobalt or iron or permalloy metalto a thickness that is sufficient to result in a conductive coating.Especially preferred are copper-coated, nickel-coated, cobalt-coated,and iron-coated lipid tubules, nickel or cobalt over copper-coated lipidtubules, and iron and nickel coatings over lipid tubules.

The methods of electroless coating are described more fully in U.S. Pat.No. 5,089,742, which is incorporated herein by reference in itsentirety.

The present composite materials typically comprise the polymer orceramic matrix in an amount of 40 to 99% by weight, preferably 48 to 99%by weight, more preferably 50 to 90% by weight, based on the totalweight of the composite material. The tubules are suitably present in anamount of 1 to 60% by weight, preferably 1 to 52% by weight, morepreferably 10 to 50% by weight, based on the total weight of thecomposite material.

Other types of materials which may be present in the composite materialinclude stabilizers (e.g., light absorbers such as carbon black and/orUV absorbers), plasticizers, fungicides, bacteriocides. Such ingredientsmay be present in any amount such that the desired level of attenuationis retained.

The composite material may further contain one or more magnetic powderedmetals and/or metal oxides, such as iron powder and/or ferrite powder.

It is also to be understood that the present composite material can bein the form of any suitable shape. Thus, when the composite material isused on the inside of a device, it may be preferred that the compositematerial have a shape which conforms to the open space(s) orinterstice(s) between components within the device or the space(s) orinterstice(s) between one or more components in the device and all orpart of a housing or casing of the device.

The present composite materials may also be applied to the outside of adevice. In this case, it may be preferred that the composite material beflexible so that it will conform to the outside of the device. Ifdesired, the rigidity or flexibility of the composite material can bemodified using conventional techniques to fit the application. In thiscase, it may also be preferred to initially form the composite materialin the form of a flexible sheet of arbitrary length and width, which maybe cut to shape at the time of application. In this type of application,the composite material will typically have a thickness of 0.025 to 2.5millimeters (mm), preferably 0.25 to 2.0 mm, more preferably 0.5 to 1.5mm.

When the composite material is to be applied to the outside of a device,it may be preferred to incorporate the composite material in a laminatedsheet, in which either one or both of the major surfaces of a sheet ofthe composite material is coated with a sheet or film of a differentmaterial. Materials suitable for coating such a sheet of the compositematerial include UV-cured acrylates and UV-cured thiol-enes. It may bepreferred that the composite material take the form of a laminate whichcontains two or more layers which contain the metal microtubules andthat the microtubules are predominately aligned in distinct directionsin at least two of the layers. Alignment of the microtubules in a layermay be carried out by flow alignment during the formation process fornon magnetic metals or by forming the matrix sheet or layer while in amagnetic field of known orientation followed by crosslinking of thepolymer matrix to lock the orientation in place.

It may also be preferred to provide at least one side of such a sheet ofthe composite material or such a laminated sheet comprising thecomposite material with an adhesive or other fastening means, such asvelcro.

In an especially preferred embodiment, the composite material forms aconformal coating which encapsulates and isolates components on aprinted circuit board. According to this object, external microwaveradiation that otherwise would be emitted or absorbed by the electroniccomponents or conductive traces on the board would be contained oreliminated.

In another preferred embodiment, the composite material comprises orconsists essentially of (a) a polyvinylbutryl or epoxy matrix; and (b)lipid or halloysite microcylinders which have been coated with a coatingof nickel, cobalt, or an alloy of nickel, iron and boron (permalloy) byelecroless deposition and in which the microcylinders are loaded intothe matrix in an amount of 0.5 to 15% by weight, based in the totalweight of the matrix and the microcylinders.

In another embodiment, the present invention provides a method formaking such composite materials, which comprises:

(1) incorporating a plurality of metal microtubules in a polymer orceramic matrix.

In this embodiment, the polymers, the ceramics, the microtubules, othercomponents, relative amounts of components, and the shape of thecomposite material are as described above.

The exact means for effecting the step of incorporating the metalmicrotubules in the polymer or ceramic matrix will depend at least inpart on the type of polymer or ceramic which forms the matrix. Forexample, if the polymer matrix is formed of a thermoplastic polymer,then the metal microtubules may be incorporated by first forming a meltof the thermoplastic polymer and then incorporating the metalmicrotubules into the melt. The microtubule-containing melt may then becooled to afford the composite material. Any suitable apparatus forforming the melt and for incorporating the metal microtubules into themelt may be used. Of course, it is to be understood that the coolingstep may be carried out in such a way to afford the composite materialin the form of a desired shape. For example, the melt may be extruded orcast into a mold or extruded as a sheet.

It is preferred that the viscosity of the melt be maintained at asufficiently high value to prevent or reduce any settling of themicrotubule while cooling in the melt. The viscosity of thethermoplastic melt may be controlled by any conventional method, such ascontrolling the temperature of the melt and/or controlling the amount ofplasticizer, if any, added to the melt.

When the polymer or ceramic matrix is formed by curing polymerprecursor(s) or ceramic precursor(s), then the metal microtubules may beconveniently incorporated into the polymer or ceramic matrix by simplyadding the metal microtubules to the polymer or ceramic precursor(s)prior to completion of the curing. Curing of the microtubule-containingmixture then affords the composite material. Once again, this step maybe carried out to afford the composite material in the form of a desiredshape. For example, the precursor components and the metal microtubulesmay be cured in a mold of the desired shape. Alternatively, the final ordesired shape may be obtained by cutting, sanding, or machining thecomposite material.

In one preferred embodiment, the polymer matrix is a resin which iscomprised of two liquid components, a base component and a curing agent.The components of the matrix resin are mixed immediately before use andwill eventually harden to form a solid matrix. A liquid mixture oftubules and freshly-mixed resin may be placed within a mold and cured.By this method, a dry, flexible matrix of defined dimensions may beformed. The composite is subsequently removed from the mold and sandedto the desired size and shape or used as is. Alternatively, the liquidmixture may be poured, or pulled by vacuum forces, directly into anelectronic device where it fills the cavities of the device.Accordingly, the mixture then assumes the shape of the internal voids asit cures. In either embodiment, the mixture cures in place withoutappreciable shrinkage.

In another embodiment, the present invention provides intermediates,which are useful for forming the present composite materials and whichcomprise:

(a) a polymer matrix precursor or ceramic matrix precursor; and

(b) a plurality of metal microtubules dispersed within said polymermatrix precursor or ceramic matrix precursor.

In this embodiment, the present intermediates, the polymer precursor(s),the ceramic precursor(s), the microtubules, other components, relativeamounts of components, and the shape of the composite material are asdescribed above. Thus, when the matrix is a polymer resin, the polymermatrix precursor may be the uncured resin. When the matrix is a ceramic,the ceramic matrix precursor may be the uncured ceramic.

In another embodiment, the present invention provides electronicdevices, which comprise a microwave-attenuating composite, saidcomposite comprising

(a) a polymer or ceramic matrix; and

(b) a plurality of metal microtubules dispersed within said polymer orceramic matrix.

In the present devices, the polymers, the ceramics, the microtubules,other components, relative amounts of components, and the shape of thecomposite material are as described above. The present devices may beany in which it is desired to attenuate microwaves originating from thedevice itself or external to the device. Examples of such devicesinclude antennas, repeaters, amplifiers, circuit boards, platformfuselages, wing tips, removable panels, multichip modules, antennas forcell phones, microwave ovens, NMRs (shielding), computers (shielding),etc.

In one preferred embodiment, the composite containing the metalmicrotubules takes the form of an article of clothing, which may beconnected to an electronic device, such as a cellular telephone. In thiscase, the article of clothing can be used as an inconspicuous antennafor the electronic device. In this embodiment, the metal microtubulesmay be contained in either electrospun polymeric fibers or in a sheet ofhighly flexible polyurethane in an amount of between 1% and 50% byweight to provide a personal communications antenna for transmissionover a range of frequencies when added to clothing to form a flexibleantenna for wear.

When it is desired to shield components within the device from oneanother, then it will be preferred to place the composite materialinside the device and between those components. When it is desired toprotect the environment (including the user(s) and other devices) fromradiation emitted from the device, then it will be preferred to fully orpartially surround the components of the device with the compositematerial. In this case, the composite material may be placed insideand/or outside any housing or casing. When it is desired to protect thedevice from radiation originating from the environment (includingambient radiation and/or other devices), then it will be preferred tofully or partially surround the components of the device with thecomposite material. In this case as well, the composite material may beplaced inside and/or outside any housing or casing.

In a particularly preferred embodiment, the device contains a printedcircuit board, and the composite material is a non-conductive polymermatrix with a suitable loading of metal microtubules (1 to 20% byweight) which forms a conformal coating. This conformal coatingencapsulates and isolates the components on the printed circuit board.In this embodiment, external microwave radiation that otherwise would beemitted or absorbed by the electronic components or conductive traces onthe board is contained or eliminated.

In another embodiment, the present invention provides a method forpreparing such devices by:

(1) incorporating a microwave-attenuating composite material in anelectronic device,

wherein said microwave-attenuating composite material comprises:

(a) a polymer or ceramic matrix; and

(b) a plurality of metal microtubules dispersed within said polymer orceramic matrix.

In this method, the polymers, the ceramics, the microtubules, othercomponents, relative amounts of components, the shape of the compositematerial, and devices are as described above.

In a particularly preferred embodiment, the device contains a printedcircuit board, and the matrix precursor is a liquid mixture of anon-conductive polymer matrix with a suitable loading of microtubules (1to 50% by weight) which is used to form a conformal coating. Thisconformal coating encapsulates and isolates components on the printedcircuit board. In this embodiment, external microwave radiation thatotherwise would be emitted or absorbed by the electronic components orconductive traces on the board is contained or eliminated.

In another embodiment, the present invention provides a method forattenuating microwaves between a source point and a detection point by:

(1) placing a microwave-attenuating composite material between saidsource point and said detection point, wherein saidmicrowave-attenuating composite material comprises:

(a) a polymer or ceramic matrix; and

(b) a plurality of metal microtubules dispersed within said polymer orceramic matrix.

In this method, the polymers, the ceramics, the microtubules, othercomponents, relative amounts of components, and shapes of the compositemay be as described above. Preferably, the method involves the use of asufficient thickness or amount of the present composite material toachieve a 10 dB (i.e., 90%) or higher attenuation of the radiation.

The metal tubule-filled composite materials described in this disclosureeffectively attenuate or absorb a broad spectrum of the radio frequencyspectrum and microwave radiation. The materials are low in density,effective at a broad range of frequencies even when very thin, able tobe cast or molded into a variety of shapes and sizes, and stable attemperatures between −20° C. and 120° C. and at any relative humidity.

These materials are suitable for use in electronic equipment on militaryplatforms because of their lightweight, high performance, and durabilityin the operating environment. Weight savings translate directly intodecreased fuel consumption and extended duty cycles.

Potential uses extend beyond commercial applications to many militaryapplications. With the technological advances in electronics, more andmore equipment and appliances are emitting electromagnetic radiation.The Federal Communications Commission has established rules andregulations to control and enforce limits on electromagneticinterference (EMI) and radio frequency interference (RFI). A microwaveoven is an example of a household device which may leak microwaves andwhich will benefit from the use of lightweight, thin microwave absorbingmaterials. Such materials may also be used in the EMI filters and lowfrequency chokes of input and output filters in switched mode powersupplies.

Prior to the present invention, it was not recognized that compositematerials with the aforementioned properties may be made from anelastomeric polymer such as an epoxy, polyurethane or siloxane, whereinthe polymer may contain hollow cylinders of a metal or metallic-coatedsubstrates measuring 1 to 100 microns long by 0.5 microns in diameter ormay contain conductive particles consisting of tubular morphologiesand/or other complementary geometric shapes.

One important feature of the use of the mixed components of elastomericresins (e.g., a polyvinylbutryl or epoxy matrix) to prepare thecomposite according to the present invention is that they have asufficiently low viscosity, and the addition of up to 14 percent byweight of tubules produces a fluid mixture which may be poured oraspirated with ease. Halloysite tubules offer the ability to use farmore energetic means of mixing the metallic tubules and the resinwithout the problem of shear-induced aspect ratio reduction.

A second important feature of the use of the mixed components of theresins described above is that they contain no volatile materials. Thus,there is no evaporation during curing and the resulting compositeclosely resembles the shape of the mold.

The mixed components may be poured into an open mold. Curing the resinproduces a solid, which conforms to the shape of the mold surfaces butmay be domed or dished on the open face.

The mixed components may be poured or forced under pressure into a mold,which is then closed. Once cured, the resulting resin is a solid, whichconforms to the exact shape of the mold.

Alternatively, the mixed components may be sucked into or forced underpressure into the internal cavities of a complex device such as acellular telephone. The resin surrounds and encapsulates the internalparts and cures in place without shrinkage.

The desired quantities of metal particles and matrix material may besimply weighed out and combined in a beaker. In order to lowerviscosities and improve mixing, the matrix materials may be warmed to40-60° C. By this method, the time available before the matrix begins tocure is reduced. Accordingly, this time must be taken into account(e.g., pot life for the commercial urethanes is usually from 30 to 90minutes). Antifoaming agents may be added to the mixture to prevent theformation of bubbles. Other additives may also be mixed in at this timeto improve either the mechanical or the electrical properties of thecomposite. In addition, small amounts of additives may be added toimprove the ease of application, storage stability, air release, tubulewetting, tubule flocculation, ease of processing, or other properties ofthe composite.

Since the quantities of metal particles may be as high as 60% percent byweight, the resulting mixtures may have very high viscosities. In thiscase, the mixing can be laborious and time consuming, and also can causebreakage of the metallized tubules. Therefore, it is preferred that theminimum amount of stirring with the minimum amount of force be used toavoid breakage.

If lower amounts of metal particulate additives are used, the mixturemay have a relatively low viscosity, which may allow the mixture to bereadily poured into a mold. Therefore, a panel may be formed in an opencontainer, i.e., without a lid. In this case, the mold must be carefullybalanced to insure uniform thickness.

When more heavily loaded mixtures with high viscosities are utilized,pouring and self-leveling are not practical options. Accordingly, themixture may be spread with a spatula in a mold. The mixture may thenplaced in a vacuum oven at about 40 to 60° C. and pumped until most ofthe bubbles are removed (about 5 minutes). Then the mold and contentsmay be covered with a lid and allowed to cure at room temperature.Spacers at the edge of the mold establish the thickness of the mixture,as it is hardening. An antistick surface may used in the mold to alloweasy demolding of the cured composite.

The present composite is a mixture of electrically conductive metalparticles in an insulating matrix. Addition of metal powder to a mixtureeventually results in sufficient metal to metal contacts to produce aconductive composite. This phenomenon is described as percolation andconstitutes a phase transition in the material. The loading with metal,which corresponds to the onset of percolation, is termed thresholdloading. Along with the onset of percolation there is also a sharpincrease in the permittivity of the composite. The “real” part of thedielectric constant increases steadily with loading density, reaching amaximum near the threshold loading. The “imaginary”, or lossy, part ofthe dielectric response increases slowly with loading, and near thethreshold jumps suddenly to large values, becoming larger than the“real” part above the threshold.

Panels of the composite can be placed on the surface separating twoantennas, one a receiving and one a sending antenna. The change incoupling between the antennas due to the tubule based film between themis termed attenuation. Attenuation is a function of the loading and thethickness of the composite. In order for the energy which couples thetwo antennas to be absorbed it must travel inside the composite,effectively being wave-guided by the composite. There is a maximumwavelength (minimum frequency) that is allowed to be guided by a layer;effectively equal to a half wavelength in the material taking intoaccount the index of refraction of the composite. Thus, to accommodatelong wavelengths, the composite panel either must be physically thick orit must have a high permittivity.

Panels made according to the present invention more typically have athickness of 1.1 to 1.4 mm with tubule loading densities of about 15volume percent and permittivities of about 30 to 50, that producedattenuation of antenna coupling of about 10 fold (10 dB) at 10 GHz.

Other features of the invention will become apparent in the course ofthe following descriptions of exemplary embodiments, which are given forillustration of the invention and are not intended to be limitingthereof.

EXAMPLES

In the following examples, and throughout this specification, all partsand percentages are by weight, and all temperatures are in degreesCelsius, unless expressly stated to be otherwise. Where the solidscontent of a dispersion or solution is reported, it expresses the weightof solids based on the total weight of the dispersion or solution,respectively. Where a molecular weight is specified, it is the molecularweight range ascribed to the product by the commercial supplier, whichis identified. Generally this is believed to be weight average molecularweight.

1. Tubule Preparation.

In order to ensure the proper final concentration sufficient foractivation of the polyion reinforced lipid, the colloidal Pd(II)catalyst dispersions suitable for use in the invention were prepared inthe following manner.

A stock 1.0 M aqueous solution of NaCl was first prepared by dissolving58.44 g NaCl (Fisher Scientific Cat. #S-271; M.W.=58.44 g/mole;Certified A.C.S. Reagent; Lot #865603) in approximately 750 mL deionized(DI) water (Nanopure II® Water Still; 17.6 MΩ resistivity). The solutionwas then diluted to 1 L with DI water. An aqueous 0.1 M morpholinoethanesulfonate (MES), pH 5 buffer stock solution was separately prepared bydissolving 2.13 g of morpholinoethane sulfonic acid (98%; AldrichChemical Cat. #16,373-2; M.W.=213.26 g/mole; Lot #EF04410EF) inapproximately 50 mL DI water and titrating dropwise with a freshlyprepared 2 M aqueous solution of NaOH (99.99%; Aldrich Chemical Cat.#30,657-6; M.W.=40.00 g/mole; Lot #LG02424KG) to pH 5.0. The resultingsolution and aqueous washings were transferred to a 100-mL volumetricflask and diluted to 100 mL with DI water to yield a stock buffersolution.

For the preparation of the metallization catalyst, 60 mg ofNa₂PdCl₄.3H₂O (98%; Aldrich Chemical Cat. #20,581-8; M.W.=294.19 g/mole;Lot #TF13323PF) was transferred to a clean 50 mL volumetric flask. A1.5-mL aliquot of the 1.0 M NaCl (aq) stock solution was added to theflask to completely dissolve the Na₂PdCl₄.3H₂O. A 10-mL portion of 0.1M, pH 5 MES (aq) stock buffer was immediately added to the flask and thesolution was diluted to the 50 mL mark with DI water. The contents ofthe flask were mixed by inversion of the flask 25 times. The flask wasthen incubated for 20 hours in a temperature-controlled water bath at23±1° C. Subsequent to incubation, a 5-mL aliquot of the resultingstraw-yellow colored dispersion was removed by pipette and discarded. A5-mL aliquot of the 1.0 M NaCl (aq) stock solution was added to thecontents of the flask by pipette and the flask was mixed by inversion asdescribed above to produce the final metallization catalyst dispersion,hereinafter referred to as PD1A.

The concentrations of Pd(II), chloride ion, and total MES in the finalpH 5 PD1A dispersion are: [Pd(II)]˜3.67 mM; [Cl⁻]127 mM; [MES]˜18 mM(approximate values). This metallization catalyst exhibited a usefullifetime of approximately 14 days at room temperature beforedestabilization and bulk precipitation of Pd(II) salts occurred. Thismetallization catalyst promoted essentially complete electrolessmetallization (i.e., >95% coverage) on more than 90% of the lipidtubules treated, as described below.

A preferred metallization catalyst for tubule metallization may beproduced with a slight modification of the above catalyst formulation.The preferred metallization catalyst formulation, hereinafter referredto as catalyst PD1B, was prepared exactly as described for catalyst PD1Awith one modification: the solid Na₂PdCl₄.3H₂O was initially dissolvedusing 1.0 mL, rather than 1.5 mL, of 1.0 M NaCl (aq) stock solution.This resulting metallization catalyst solution was a dark yellow-brownto orange-brown color after preparation. The concentrations of Pd(II),chloride ion, and total MES in the final pH 5 PD1B dispersion are:[Pd(II)]˜3.67 mM; [Cl⁻]˜118 mM; [MES]˜18 mM (approximate values).Although the useful lifetime of this metallization catalyst beforedestabilization and bulk precipitation of the Pd(II) salts is reduced toapproximately 10 days at room temperature, use of PD1B catalyst leads toessentially complete electroless metallization (i.e., >95% metalcoverage) on more than 95% of the treated lipid tubules, as describedbelow.

The advantage of using these palladium metallization catalysts (i.e.,PD1A or PD1B) to catalyze the electroless metallization of the lipidtubules over other methods using Pd/Sn catalysts (see, e.g., U.S. Pat.No. 6,013,206) is that it (1) eliminates the need to remove thealcoholic solvent mixture prior to the electroless metallization step bydialysis, (2) eliminates the need for acidic salts that tend to disruptthe structure of the microtubules, and (3) eliminates the need toconduct an activation step for the stannous palladium commercialcatalyst system used in previous methods as elucidated in U.S. Pat. No.6,013,206, which is incorporated herein by reference herein. Anotheradvantage is that the number of filtrations (which are typically usedafter each step in the Pd/Sn catalysis method) is reduced. As few as onefiltration to remove the PD1A or PD1B metallization catalyst isrequired. This is beneficial, because filtrations are the source of muchof the breakage of the tubules, which is detrimental to the intendeduse.

In another embodiment, halloysite, a clay which occurs naturally ashollow cylinders, may be metallized by first treating the surface of theclay with an active metallization catalyst as described above. In thiscase, the clay need not be withdrawn from the center of the metallictubule before use.

Washing and pin milling until a reduced particle size is achieved may becarried out to first prepare the clay. It is preferred that the clay beprepared from a deposit of halloysite that is comprised of essentiallyall tubular materials. These materials are blunged to reduce theparticle size and washed with Calgon salts to remove any exogenousallophate materials. This is followed by the Pd(II) catalysis stepoutlined above, which entails simple suspension of the clay at a nearneutral pH followed by the addition of the aqueous palladiummetallization catalyst. Subsequently, centrifugation or filtration mayremove the excess catalyst and the clay is resuspended in water. Theprocess is repeated until all unbound catalyst is removed. Followingremoval of the catalyst, the halloysite is capable of being electrolessplated by the same process as the lipid microtubles. The halloysite clayis available in a range of sizes up to an aspect ratio of 40-50.Specific note may be made of the following: Copper Coated, NickelCoated, Cobalt Coated, or Iron Coated halloysite tubules, Nickel orCobalt over Copper coated halloysite tubules, and Iron, Nickel and Boronalloy coatings over halloysite tubules. The halloysite materials aredescribed more fully in U.S. Pat. No. 5,651,976, which is incorporatedherein by reference in its entirety.

Elastomeric Matrix Resin Example 1: Elastomeric Polyurethane MatrixResin

A polyether diol, a polyether triol, metal-plated tubules and a curingcatalyst are mixed and then cured with a diisocyanate prepolymercontaining a plasticizer. In a preferred embodiment, a base componentand a curing agent are made separately as follows:

Base Component: Mix thoroughly the following:

Diol: 2.54 g, poly(propylene glycol) [C. A. 25322-69-4], having aweight-average molecular weight of about 425, a viscosity of 80centistokes at room temperature, and a density of 1.004 g/mL.

Triol: 0.41 g, trimethylolpropane propoxylate [C. A. 25723-16-4], havinga molecular weight of 308 and a density of 1.040 g/mL.

Curing Catalyst: 0.03 g, dibutyltin dilaurate [C. A. 77-58-7], having amolecular weight of 631 and a density of 1.066 g/mL.

Metallized Tubules: metal microtubules with a wall thickness of 10 to2000 nm or halloysite microcylinders with a metallic coating thicknessof 10 to 1000 nm are mixed to a final weight percentage of 1 to 50percent of the base component with loadings from 10 to 20 percentpreferred.

Curing Agent: Mix thoroughly the following:

Diisocyanate prepolymer, 9.60 g, poly(propylene glycol) terminated withtoluene-2,4-diisocyanate [C. A. 9057-91-4], having a weight-averagemolecular weight of about 1000, a viscosity of 12,500 centipoise at 40°C., and an average isocyanate content of 8.4 weight percent.

Plasticizer, 0.24 g, dibutyl adipate [C. A. 105-99-7], having amolecular weight of 258 and a density of 0.962 g/mL.

The base component and curing agent are then blended togetherthoroughly, poured into a mold, and allowed to cure at room temperature.In this manner a tough, elastomeric composite with a Shore A Hardness of52 is produced.

Elastomeric Matrix Resin Example 2: Elastomeric Polyurethane MatrixResin

The elastomeric matrix resin may be made with a commercial polyurethaneelastomer as the base component. A suitable product is LS-40 from BJBEnterprises, Inc. (14791 Franklin Avenue, Tustin, Calif. 92780).According to this method, 90 g of the base component is mixed with thecuring agent (100 g), poured into a mold, and allowed to cure at roomtemperature. The resulting resin has a Shore A hardness of 40, a tensilestrength of 490 pounds per square inch, and an elongation of 800%.

Elastomeric Matrix Resin Example 3: Elastomeric epoxy matrix resin. Thediglycidyl ether of bisphenol A, diluted with a monoepoxide, is mixedwith a polyglycol diglycidyl ether and cured with apolyoxypropylenediamine. In a preferred embodiment, a base component anda curing agent are made separately as follows:Base Component: Mix thoroughly the following:

A liquid epoxy resin (46 g) containing the diglycidyl ether of bisphenolA and butyl glycidyl ether. The mixture has a viscosity of 5-7 Poise, aspecific gravity of 1.13, and an epoxy equivalent weight of 175-195.

A polyglycol diepoxide resin (31 g) having a viscosity at 25° C. of30-60 centipoise, a specific gravity at 25° C. of 1.14, and an epoxyequivalent weight of 175-205.

Curing Agent

A polyoxypropylenediamine (23 g) having a viscosity at 25° C. of 9centipoise, a specific gravity at 20° C. of 0.948, and an averagemolecular weight of 230.

The base component and curing agent are then thoroughly blendedtogether, poured into a mold, and allowed to cure at room temperature. Atough, elastomeric resin with a Shore A hardness of 65 is obtained.

Elastomeric Matrix Resin Example 4: Elastomeric siloxane matrix resin. Avinyl-terminated polydimethylsiloxane and a copolymer ofmethylhydrosilane and dimethylsiloxane are mixed with a curing catalyst.In a preferred embodiment, a base component and a curing agent are madeseparately as follows:Base Component: Mix thoroughly the following:

A vinyl-terminated polydimethylsiloxane, 50 g, having a molecular weightof 28,000, a viscosity of 1000 centistokes, a specific gravity of 0.97,and containing 0.18 to 0.26 weight percent of vinyl groups.

A curing catalyst solution, 15 microliters, formed from 39 parts ofn-hexane and one part of 1,2-divinyltetramethyldisiloxane containing 3.0to 3.5 weight percent of platinum, which has a viscosity of 0.7centistokes and a specific gravity of 0.81. Further, one milliliter ofthis solution contains one milligram of platinum.

Curing Agent

A copolymer of methylhydrosilane and dimethylsiloxane, 1.5 g, having amolecular weight of 1900-2000, a viscosity of 25-35 centistokes, aspecific gravity of 0.98, and containing 25-30 mole percent ofmethylhydrosilane moieties.

The base component and curing agent are thoroughly blended together,poured into a mold, and allowed to cure at room temperature. By thismethod a tough, elastomeric composite is produced.

Elastomeric Matrix Resin Example 5: Elastomeric siloxane matrix resin.The elastomeric matrix resin may be made using a commercial siloxaneelastomer. Smooth-Sil 900 from Smooth-On, Inc. (2000 St John Street,Easton, Pa. 18042) is suitable for this purpose. According to thismethod, 100 g of the base component is mixed with the curing agent (100g), poured into a mold, and allowed to cure at room temperature. Theresulting resin has a Shore A hardness of 35, an elastic modulus of 77,and an elongation of 175%.

Obviously, numerous modifications and variations on the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art of polymers. Although methods and materials similar orequivalent to those described herein can be used in the practice ortesting of the present invention, suitable methods and materials aredescribed herein. All publications, patent applications, patents, andother references mentioned herein are incorporated by reference in theirentirety. In case of conflict, the present specification, includingdefinitions, will control. In addition, the materials, methods, andexamples are illustrative only and are not intended to be limiting.

1.-19. (canceled)
 20. A process for producing a metallic lipidmicrotubule comprising, (a) suspending a lipid in an alcohol-watersolvent, to form lipid microtubules; (b) adding a polyanion mixture, toobtain a lipid microtubule-polyanion mixture; (c) incubating said lipidmicrotubule-polyanion mixture, to obtain an incubated lipidmicrotubule-polyanion mixture; (d) adding a polycation mixture to saidincubated lipid microtubule-polyanion mixture, to obtain apolycation-lipid microtubule-polyanion mixture; (e) adding a palladiumcatalyst to said polycation-lipid microtubule-polyanion mixture, toobtain a catalyst mixture, wherein said palladium catalyst is selectedfrom the group consisting of PD1A and PD1B; (f) incubating said catalystmixture for a minimum of 30 minutes; and (g) electroless coating thelipid microtubules.
 21. The process according to claim 20, wherein saidlipid is suspended at a concentration of 2.5 mg/ml.
 22. The processaccording to claim 20, wherein said alcohol-water solvent comprises 70wt. % of an alcohol mixture and 30 wt. % distilled water.
 23. Theprocess according to claim 22, wherein said alcohol mixture comprises 20to 80 wt. % methanol and 20 to 80 wt. % ethanol.
 24. The processaccording to claim 20, wherein said polyanion mixture has aconcentration ranging from 0.1 to 1.0 mg/ml.
 25. The process accordingto claim 20, wherein said polyanion mixture comprisespolystyrenesulfonate.
 26. The process according to claim 20, whereinsaid incubating said lipid microtubule-polyanion mixture is carried outfor 15 to 45 minutes.
 27. The process according to claim 20, whereinsaid polycation mixture has a concentration ranging from 0.5 to 1.5mg/ml.
 28. The process according to claim 20, wherein said polycationmixture comprises polyethyleneimine or chitosan.
 29. The processaccording to claim 20, wherein the said incubating results in theformation of a cross-linked polycation layer on the surface of saidlipid microtubules.
 30. The process according to claim 20, wherein saidPD1A is added in the form of a solution or dispersion which comprises3.5 to 3.75 mM [Pd(II)], 120 to 130 mM [Cl⁻], and 15 to 20 mM [MES] at apH of
 5. 31. The process according to claim 30, wherein said solution ordispersion comprises about 3.67 mM [Pd(II)], about 127 mM [Cl⁻], andabout 18 mM [MES] at a pH of
 5. 32. The process according to claim 20,wherein said PD1B is added in the form of a solution or dispersion whichcomprises 3.5 to 3.75 mM [Pd(II)], 115 to 120 mM [Cl⁻], and 15 to 20 mM[MES] at a pH of
 5. 33. The process according to claim 32, wherein saidsolution or dispersion comprises about 3.67 mM [Pd(II)], about 118 mM[Cl⁻], and about 18 mM [MES] at a pH of
 5. 34. The process according toclaim 20, wherein said palladium catalyst is added in the form of asolution or dispersion at a volume ratio of 1:500 of said solution ordispersion to said polycation-lipid microtubule-polyanion mixture. 35.The process according to claim 20, wherein said electroless coatingcomprises adding a formulation comprising a metal selected from thegroup consisting of copper, nickel, cobalt, iron, and a permalloy metal.36.-45. (canceled)